In the field of sampling and analysis, there is extensive prior art. Most descriptions, however, give detailed guidelines for analyzing the samples, with special emphasis on the physical and/or chemical principles of the analysis and the measuring technique, and the manner of sampling and sample treatment comes second.
It is often desirable to monitor the composition of a fluid stream with reference to a number of criteria during a certain period of time. Examples of such fluid streams are liquids, such as product streams in the process industry, but also gas streams, such as natural gas or flue gases. To monitor the composition, a sample is taken from the fluid stream at certain intervals or continuously, which is subsequently analyzed. In the analysis, one or more parameters must not exceed pre-determined maximum values, and often must not fall below certain minimum values either. Also, it is often desirable to record the amount of the maximum and minimum values which occur or to keep samples corresponding to such values.
Such requirements imposed upon the fluid stream may be imposed by the Government, in the form of legal stipulations (such as environmental requirements, for example, emission values) or may be the result of agreements between producer and buyer with regard to the quality of products supplied. Also, the requirements may be quality guidelines applying within a concern. The parameters in question invariably concern intensive magnitudes, such as chemical composition (concentration), viscosity, optical rotation, etc.
In order that the composition may be monitored, it is necessary to take a sample from the fluid stream, which is supplied to an analyzer. These samples should of course be representative of the composition of the entire fluid stream.
Sampling can be effected in two ways: by random testing or (semi)-continuously. This invention relates to a method in which samples are taken semi-continuously. In it, a sample is taken from the fluid stream at certain intervals, which sample is analyzed to determine the desired parameter value.
One disadvantage of such a method is, however, that the taking of the sample and the subsequent analysis require relatively much time. As a consequence, there is the risk that the parameter value in the fluid has meanwhile been changed by the time the analysis is completed. Generally speaking, therefore, the means will fail to give an adequate impression of the entire fluid stream. The solution of these problems has hitherto only been sought in the use of ever faster, and hence ever more expensive, measuring methods. Generally speaking, however, speed goes to the detriment of accuracy, reliability and quality.
Another disadvantage of existing methods turns out when the analysis has revealed a parameter value that is outside the range permitted. In that case there are two possibilities:
(a) the measurement is faulty PA1 (b) the extreme parameter value has indeed been exceeded.
As the sample has been used, it can no longer be determined which of the two possibilities presented itself. It is desirable, therefore, that an additional sample is available, which is equal to the sample analyzed.
Sometimes it is desirable for other reasons that a sample of the fluid stream is kept when (one of) the analysis-parameters assume(s) an extreme value. By means of such samples it can thus be shown that the composition of the fluid stream has been within the legally or contractually required, or desired limits within a given period of time.